1. Field of the Invention
The present invention relates to a squarylium dye having absorption in the near infrared part and a method of producing the same. The present invention also relates to a photoelectric conversion element having a photoelectric conversion part containing a pair of electrodes and an infrared organic photoelectric conversion film which contains the squarylium dye and is provided between the pair of electrodes, and a solid-state imaging device.
2. Background Art
With recent progress of organic electronics, an element using an organic dye thin film has been involved in intensive development. As one of these elements, an element using an organic material for the photoelectric conversion film is actively studied, typically including an electrophotographic device and a solar cell, and studies on various materials therefore are being made. For example, the electrophotographic material includes those described in Kock-YeeLaw, Chem. Rev., Vol. 93, p. 449 (1993), and the material for a solar cell includes those described in S. R. Forrest, J. Appl. Phys., Vol. 93, p. 3693 (2003). However, the materials described in either publication are in principle unable to favor wavelength selectivity, because the film formed has a broad absorption spectrum and the photoelectric conversion spectrum indicative of the wavelength dependency of photoelectric conversion ability becomes broad. In particular, a material suitable for the vapor deposition process that is indispensable in producing an organic electronic element, and assured of strong absorption in the near infrared region and no absorption in the visible part has been heretofore not obtained.
A squarylium dye has been intensively studied as an electrophotographic material because of its characteristic sharp absorption spectrum and good photoelectric conversion properties, but satisfactory control of the absorption spectrum is not possible. For example, a squarylium dye capable of being vapor-deposited is disclosed in Seok Hwan Hwang, et al., Dyes and Pigments, Vol. 39, p. 359 (1998), but this material has absorption in a region of short wavelength, failing in satisfactorily utilizing infrared light, and has strong absorption in the visible light region. In Jian-Guo Chen, et al., Dyes and Pigments, Vol. 46, p. 93 (2000) and JP-A-2006-106469 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), dyes having small visible absorption and having strong absorption in the near infrared region are disclosed, but their vapor deposition property is insufficient.
Conventional visible light sensors in general are produced by forming a photoelectric conversion element through formation of PN junction in a semiconductor such as Si. As for the solid-state imaging device, there is widely used a flat-type image-receiving device where photoelectric conversion elements are two-dimensionally arrayed in a semiconductor and a signal generated resulting from photoelectric conversion is read out by each photoelectric conversion element according to a CCD or CMOS format. The method for realizing a color solid-state imaging device is generally a structure where a color filter capable of transmitting only light at a specific wavelength is disposed for color separation on the light incident surface side of the above-described flat-type image-receiving device. Particularly, a single-plate sensor in which color filters capable of transmitting blue light, green light and red light are regularly disposed on respective two-dimensionally arrayed photoelectric conversion elements is well known as a system widely used at present in a digital camera and the like.
The sensor sensitive to infrared light generally uses a heat-type sensor (e.g., thermal electromotive force type, current collection effect, thermocouple effect) or a quantum-type sensor (e.g., photovoltaic effect, photoconductive effect, photoemission effect). Most of these sensors are composed of an inorganic semiconductor and since the inorganic semiconductor has broad absorption at a wavelength shorter than a certain wavelength, these sensors have a property of absorbing light in the entire region from the infrared region to the visible region.
In the case of simultaneously obtaining an image of visible light and an image of infrared light, there may be considered a method of separating the incident light into infrared light and visible light and detecting these lights by different devices, and a method of two-dimensionally disposing color filters capable of transmitting visible light and infrared light, respectively, on one device. According to such a method, an image of visible light and an image of infrared light may be obtained at the same time, but these methods have problems, for example, that: the size of the device or apparatus becomes large and the cost rises; the image is not sampled at the same point and the synthesis, processing and the like of image information are difficult; or the color filter is transparent only to light at a limited wavelength and the non-transmitted light is not utilized, giving rise to bad light utilization efficiency.
For solving these problems, a method of stacking photoelectric conversion parts capable of detecting lights at different wavelengths may be considered. As regards such a system, in so far as visible light is concerned, for example, U.S. Pat. No. 5,965,875 discloses a sensor in which a vertical stack structure is formed inside of a silicon substrate to utilize the wavelength dependency of absorption coefficient of Si and the color is separated by the difference among respective depths, and JP-A-2003-332551 discloses a sensor having a structure where an organic photoelectric conversion film is stacked on the upper side of a silicon substrate.
The system of stacking, in the vertical direction, a photoelectric conversion part for detecting infrared light and a photoelectric conversion part for detecting visible light is disadvantageous not only in that the absorption ranges of respective portions are overlapped in the depth direction of the silicone substrate to give bad spectral characteristics and the color separation is originally poor, but also in that although it is necessary in the case of using Si to further provide a photoelectric conversion part for detecting infrared light as a lowermost layer of the silicon substrate in the sensor of U.S. Pat. No. 5,965,875, infrared light is absorbed by the upper layer to reduce the infrared light reaching the lowermost layer inside of the silicon substrate and the sensitivity decreases.
As described above, when an inorganic semiconductor is used, the inorganic semiconductor alone can be hardly made to absorb only infrared light, but an organic film can be designed to absorb only light in a specific wavelength region and therefore, can be used as a layer which absorbs only infrared light.
The method of forming the organic film includes, for example, a coating method such as spin coating, and a vapor deposition method of vaporizing a material under heating in vacuum and depositing it on a base, and in view of preventing intermingling of impurities and favoring wide latitude in forming multiple layers so as to achieve high functionality, a vapor deposition method is preferred. In this case, for example, a chroconium or merocyanine-based dye representative of a dye having absorption in the infrared region has a low decomposition temperature and readily decomposes due to heating during vapor deposition and a film can be hardly formed. As for the known material having absorption in the infrared region, which can be vapor-deposited and is proved to exhibit a high photoelectric conversion function in an electrophotographic device, organic thin-film solar cell or the like, there is reported a phthalocyanine-based dye in JP-A-63-186251. However, according to the studies by the present inventors, use of a phthalocyanine-based material is disadvantageous in that a high photoelectric conversion efficiency can be hardly exerted in a wide wavelength region.
Other than these materials, a squarylium-based dye is known in Applied Physics Letters, 29, 414 (1975) as a material which can be vapor-deposited and exhibits a significant photoelectric conversion performance. The squarylium-based dye generally decomposes at the vapor deposition and therefore, it is difficult to produce an element having high performance. A system using a squarylium-based dye in a vapor-deposition system is known (Dyes and Pigments, Vol. 39, No. 4, p. 359 (1988)), but application of this system is limited to an electrophotographic system.